1. Technical Field of the Invention
The present invention relates to electrical apparatus, and more particularly, to electrical apparatus, such as robots and PLCs (programmable logic controllers), having higher durability against surges and being fixedly used in a stationary plant.
2. Related Art
Electrical apparatus is required to have durability against instantaneous voltage changes, i.e. surges (a sharp rise in voltage), due to lightning, for example. Therefore, electrical apparatus has been ensured to have durability against overcurrent that accompanies surges by providing a protection circuit in an electric circuit (e.g., refer to JP-A-H01-049983). Such a protection circuit is required to be provided to each of the electric circuits in electrical apparatus. In other words, one electric circuit requires one protection circuit. For this reason, in the case where electrical apparatus has a plurality of input terminals for the supply of current from a power source, i.e. in the case where the electrical apparatus includes a plurality of electric circuits, protection circuits are required to be provided to the electric circuits having the respective input terminals.
Robots or PLCs, for example, are categorized as electrical apparatus for fixed facilities and thus are used by being fixed in a plant. There has been a need, recently, for reducing the size of such electrical apparatus by not only mechanically reducing the size of its case (as well as its thickness), for example, but also reducing the size of the board and the electric circuits accommodated in the case. FIG. 10 is a schematic diagram illustrating a circuit configuration of conventional electrical apparatus. The size reduction of electrical apparatus has been attempted by closely disposing two or more electric circuits, e.g. electric circuits 101 and 102 shown in FIG. 1, in parallel and by improving the density.
In this case, in order to reliably absorb the surge energy in the individual electric circuits 101 and 102, the capacities (resistance) of resistance elements 103 and 104, respectively, are required to be set to a little larger than the capacity levels corresponding to the expected surges. However, as the capacities of the resistance elements 103 and 104 are made larger, the sizes of these elements are needed to be proportionately increased. Therefore, enhancing the durability against surges may problematically necessitate the increase of the size of the resistance elements 103 and 104, and accordingly, the size of the electric circuits 101 and 102, respectively. Further, disposing a plurality of electric circuits, such as the electric circuits 101 and 102, in parallel may induce unexpected irregular discharges between the adjacently located electric circuits 101 and 102 when high voltage is applied with a surge. The irregular discharges may result in causing damage to the electric circuits 101 and 102.
Taking the circuit configuration shown in FIG. 1 as an example, let us assume that discharge has occurred between a terminal 107 of a light emitting diode 106 of a functional circuit 105 in the electric circuit 101 and a terminal 108 in the adjacent electric circuit 102. In this case, overcurrent induced by the surge may pass through the light emitting diode 106 of the functional circuit 105 and may be likely to electrically break the light emitting diode 106. It is also assumed that discharge occurs between a coupler 109 of the electric circuit 101 and a coupler 110 of the electric circuit 102. In this case as well, overcurrent induced by the surge may pass through the light emitting diode 106 of the functional circuit 105 and may likely to electrically break the light emitting diode 106. In addition, if discharge occurs between a coupler 111 of the electric circuit 101 and a coupler 112 of the electric circuit 102, overcurrent induced by the surge may pass through the light emitting diode 106 as well as the couplers 111 and 112 and may be likely to electrically break these elements.